Recently, a large capacity (Wh) secondary battery has been developed as a power source of a hybrid electric vehicle, a pure electric vehicle, or the like. Among secondary batteries, a lithium-ion secondary battery which has high energy density (Wh/kg) and a rectangular shape has attracted attention.
In the rectangular lithium-ion secondary battery, a flat wound electrode group is formed as a power generation element. The flat wound electrode group is obtained by stacking a positive electrode, a negative electrode, and a separator and winding a result of stacking. The positive electrode is obtained by coating a positive electrode foil with a positive electrode active material. The negative electrode is obtained by coating a negative electrode foil with a negative electrode active material. The separator is used for insulating each of the positive electrode and the negative electrode. The wound electrode group is electrically connected to a positive external terminal and a negative external terminal which are provided on a battery lid of a battery case. The wound electrode group is stored in the battery can of the battery case and an opening of the battery can is welded and sealed to the battery lid. The rectangular secondary battery is formed in such a manner that an electrolyte is injected through a liquid injection hole of the battery case which stores the wound electrode group and then a liquid stopper is inserted, and sealing and welding are performed by laser welding.
Positive external terminals and negative external terminals of a plurality of rectangular secondary batteries are electrically connected by using a conductive member such as a bus bar, and thus a battery pack is formed. The bus bar is connected to the rectangular secondary battery by performing screw-fastening to the external terminals with a bolt and a nut, or by performing welding to the external terminals.
PTL 1 discloses a secondary battery in which the bus bar can be connected to an external terminal by performing welding. The secondary battery disclosed in PTL 1 includes a current collector (current collecting member in PTL 1) which is bonded to the power generation element, and a connection member (caulking member) which is disposed on the battery lid and is electrically connected to an external terminal. In the secondary battery disclosed in PTL 1, one end of the connection member and the external terminal are fixed by caulking, and another end of the connection member and the current collector are fixed by caulking.